The invention relates to the field of civil aviation and, more specifically, relates to the flight management systems, better known by the English acronym FMS, with which all civilian aircraft are nowadays equipped. An FMS is made up of various functional components that enable the crew of an aircraft to programme a flight from a navigation database. The FMS calculates lateral and vertical trajectories enabling the aircraft to reach its destination. These calculations are based on the characteristics of the aircraft, on the data supplied by the crew and on the system environment. The positioning and guidance functions then work together in order to enable the aircraft to remain on the trajectories defined by the FMS.
By improving the guidance speedwise of the aircraft, the invention aims to improve the possibilities of said aircraft in order to enable them to reach particular points at a required time, with a maximum of accuracy. This need devolves from the exponential increase in air traffic and the corresponding workload for air travel controllers. Thus, for safety reasons, but also for reasons of economic viability, it becomes essential to impose time constraints on the aircraft, notably in the approach phase. The aircraft are thus required to reach a particular waypoint in their flight plan at a required time. These particular points may be a landing runway threshold, a point of convergence of flows of aircraft, particularly busy crossing points, etc. This can make it possible, for example, to smooth the flow of aircraft before the approach phase.
These days, the FMS of an aircraft needs to handle the function of keeping to a time constraint, in other words it calculates optimized flight parameters, in order to reach particular points of the flight plan at precise times, in the most effective way possible and, for example, economically.
In order to keep to this time constraint, the FMS defines a speed strategy.
Hereinafter in the description and in the claims, the expression “speed strategy” will be understood to mean a speed profile assumed to have to be followed by the aircraft, the a priori function of the guidance module being to determine at any instant in the flight a setpoint speed, that the aircraft seeks to reach, aiming to observe said speed strategy.
Currently, the FMS of an aircraft consequently carries out prediction calculations in order to observe a required time of arrival at a particular waypoint of the flight plan, the time being commonly designated by the English acronym RTA (Required Time of Arrival); this requires it to determine the speed strategy. The FMS regularly calculates an estimated time of arrival at said particular waypoint, the time being commonly designated by the English acronym ETA (Estimated Time of Arrival). If the estimated time of arrival departs from a predetermined tolerance relative to the required time of arrival, a new cycle of calculations takes place, causing the FMS to redefine the trajectories to be followed by the aircraft, as well as the speed strategy.
Hereinafter in the description and in the claims, the expression “required time of arrival” will logically be understood to mean a time at which the aircraft must reach a particular point in its flight plan. The expression “estimated time of arrival” will be understood to mean a time at which the FMS of the aircraft predicts reaching said particular point, taking into account the current speed of the aircraft and weather conditions for example.
The defining of the speed strategy of an aircraft is an iterative process: on each loop, the FMS recalculates the trajectories to be followed, taking into account, for example, wind estimation models, and predicts an estimated time of arrival. The aim is to achieve convergence between the estimated time of arrival and the required time of arrival. This regular redefinition of the speed strategy is made necessary by the unavoidable calculation inaccuracies, due notably to the imperfections of the models. Thus, it is not a matter of control, but of reoptimization in open loop mode, performed at regular intervals.
Each calculation loop, comprising the calculation of trajectories and the definition of the speed strategy of the aircraft, requires a relatively long time, given the algorithmic complexity of said calculations to be made.
The tolerance with respect to the required time of arrival is generally modelled in the form of a funnel, that is to say that it is increasingly narrow as the aircraft approaches the particular waypoint. In practice, on approaching said particular waypoint, meeting the required time of arrival, that can be called RTA, demands increasing accuracy.
Now, each aircraft has a speed envelope, comprising a maximum speed profile and a minimum speed profile, said maximum and minimum speeds being able to vary according notably to the altitude and mass of the airplane. This speed envelope defines a speed envelope that can be achieved by the aircraft; the current FMSs therefore define the speed strategy within this speed envelope.
Moreover, the aircraft has a guidance module, notably comprising a speed guidance component. In current aircraft, the speed guidance conforms to the speed strategy calculated by the FMS.
The problem lies in the fact that the FMS of an aircraft has uncertain parameters, since the models lack accuracy, that prevent it, since the interpolation models are also imperfect, from determining a speed strategy that can be ensure that the time constraint is kept to in a certain manner.
Worse, the limitations of the speed envelope added to the inaccuracies of the prediction models may cause the FMS to predict keeping to the RTA up to a point where it has become impossible to correct a drift that is observed all of a sudden, for example following a change of wind conditions.
Currently, aircraft operators tend to give themselves a manoeuvring margin enabling them to keep to their time constraints. However, this empirical approach does not provide an adequate guarantee; furthermore, it tends to induce changes of speed of significant amplitude, that are sources of discomfort for the passengers and of excess fuel consumption.
Finally, the complexity of the calculations and the increasing uncertainty as the aircraft approaches the particular point at which it should arrive at a required time bring about the deactivation of the iterative systems for determining the trajectories and the speed strategy, for example three minutes before the time of arrival at said particular point.
One aim of the invention is notably to overcome the abovementioned technical drawbacks. Thus, to ensure that an aircraft keeps to a time constraint, while limiting its complete changes of speed strategies, the present invention proposes adjusting the speed strategy through the intermediary of the speed guidance function of the aircraft, a function to which a manoeuvring margin is granted, the dynamic for calculating the speed setpoint used for the guidance being much faster than that induced by a complete calculation of the trajectories and of the speed strategy.